The current state of the art in implementing transpulmonary thermodilution measurement are apparatus for injecting a bolus of thermal indicator into a patient's vena cava superior, and measuring the temperature response at a place of the patient's systemic circulation, e.g. patient's femoral artery to determine the thermodilution curve, i.e. the temperature response as a function of time. From the thermodilution curve, a schematic example of which is illustrated in FIG. 3, wherein the abscissa (time axis) 1 is linear and the ordinate (temperature difference axis) 2 is logarithmic, various cardio-vascular parameters can be derived by using computer systems running computer programs, which implement parameter calculations as disclosed in WO 93/21823, the contents of which are included herein by citation, and as set forth briefly below.
The Cardiac Output CO can be determined by algorithms based on the Stewart-Hamilton-equation:
  CO  =                              V          L                ⁡                  (                                    T              B                        -                          T              L                                )                    ⁢              K        1            ⁢              K        2                    ∫                                    T            B                                      Δ                                  ⁡                      (            t            )                          ⁢                                  ⁢                  ⅆ          t                    where TB is the initial blood temperature, TL is the temperature of the liquid bolus, which is used as thermal indicator, VL is the thermal indicator volume, K1 and K2 are constants to consider the specific measurement setup, and ΔTB(t) is the blood temperature as a function of time with respect to the baseline blood temperature TB. Thermal indicator can either be colder or warmer with respect to blood temperature. To obtain cardiac output, the area under the thermodilution curve has to be determined by mathematical integration.
Other parameters that can be derived from the thermodilution curve 3 as schematically illustrated in FIG. 1 include the Exponential Decay or Downslope Time DST, i.e. the time the blood temperature difference ΔTB(t) takes to drop by the factor e−1, the Appearance Time AT, i.e. the time span between bolus injection IT and first appearance of a noticable temperature difference ΔTB(t) and the Mean Transit Time MTT.
The Intrathoracic Thermovolume ITTV and the Intrathoracic blood volume ITBV can be determined as follows:ITTV=CO·MTT ITBV=a′·GEDV+b′wherein a′ and b′ are species-specific constants and GEDV is the Global End-Diastolic Volume, which can be determined as follows:GEDV=CO·(MTT−DST)
An extravascular thermovolume estimate can be determined as the difference between Intrathoracic Thermovolume ITTV and the Intrathoric blood volume ITBVETV=ITTV−ITBV 
Extravascular thermovolume correlates, if there is no significant perfusion defect in the lungs (e.g. massive pulmonary embolism or large single embolism), closely to the degree of Extravascular Lung Water.
Transpulmonary thermodilution has been shown to be a reliable technique for assessing cardiac output, cardiac preload and extravascular lung water (EVLW), i.e. to quantify pulmonary edema. The estimation of EVLW by the injection of a single thermal indicator is based on the above mentioned relationship ITBV=a′·GEDV+b′. This method has been shown to compare favorably with the double-indicator (thermo-dye) dilution technique and with the ex-vivo gravimetric methods.
However, for mechanically ventilated patients and patients suffering from severe pulmonary edema the results were not entirely satisfactory.
It is therefore an object of the present invention to provide a new apparatus, a new computer system and a new computer program allowing the determination of the intrathoracic blood volume by single indicator transpulmonary thermodilution with enhanced accuracy especially for patients suffering from severe pulmonary edema and/or for mechanically ventilated patients.